1. Field of the Invention
The present invention relates to a sliding piston for a syringe, which is slidably inserted in a hollow barrel to inject a liquid medicament contained in the hollow barrel into a human or animal body.
2. Description of the Related Art
A syringe is essentially composed of a hollow barrel (tubular body), a sliding piston and a plunger. Upon injection of a liquid medicament, the sliding piston is moved by the plunger to inject the liquid medicament contained in the hollow barrel. The sliding piston must be tightly fitted in the hollow barrel to be liquid-tight and air-tight fashion so as to smoothly slide therein even when the sliding piston is moved forward or rearward in the hollow barrel held in an inclined angle. The hollow barrel is usually made of glass or plastic. In general, the glass barrel is used also as a container for a liquid medicament, and the plastic barrel is used for a single-use type or disposable type syringe. In either barrel, the inner diameter, linearity, surface smoothness, and the roundness, of the hollow barrels are inevitably irregular to some extent due to manufacturing error.
The sliding piston is usually made of a resilient material, such as rubber or thermoplastic elastomer, to absorb the irregularity in the shape of the hollow barrel. The conventional resilient piston (rubber bulb) is in the form of a simple cylinder, a truncated cone whose diameter is gradually decreased toward the rear end, or a cylinder or truncated cone with annular projections at the front and rear ends thereof. However, according to the inventor's analysis, the conventional resilient piston can be still improve its physical properties, and in particular, slidability and air-tightness.
In particular, in a laminated piston proposed by the assignee of the present application, in which the surface of the resilient piston is coated with a laminated layer selected from a group of tetrafluoroethylene, ethylene-tetrafluoroethylene and supermacromolecular polyethylene resin films, the elastic compliance of the resilient piston tends to be reduced due to the laminated layer. To solve this problem, the physical properties of the sliding piston must be improved. The proposed laminated piston is of a silicone-free type in which it is not necessary to coat the sliding portion with a silicone oil layer as a lubricant. In the conventional sliding piston which is coated with the silicone lubricant, there is a possibility that the silicone oils mix with the liquid medicament and contaminate the liquid medicament with the particulate.